Automatic switching device for the power source input range of a monitor used in a personal computer

ABSTRACT

An automatic switching device for the power source input range of a personal computer monitor is installed within the switchmode power source circuit of the monitor and includes a comparator and a relay arranged so that, irrespective of whether the power source is at a 110V or 220V voltage level, the rectifying circuit of the switchmode power source circuit is automatically switched to a voltage doubler circuit or a bridge rectifier circuit as appropriate for reducing power losses and saving energy.

BACKGROUND OF THE INVENTION

The present invention relates to an automatic switching device for the power supply input range of a monitor connected with a personal computer, and especially to an automatic range switching device of a low loss rapidly restoring power supply commonly used in voltages of 110V and 220V.

According to the statistics, nowadays at least 250,000,000 monitors of personal computers are used. Assume the average power consumption of each monitor is 50V, the total power consumption will be a very great number. Thus many consumer organizations and energy management institutions have called for reducing the power consumption.

In order to be conveniently manufactured and sold, and to prevent the consumer from making a mistake while using, the specifications of the globally used monitors are called “Auto Range”: AC110V±20% and AC220V±20%, or Full Range: AC86V˜264V. This wide range of power supply design can't meet the purpose of using the least power of the monitors. The following table lists the power circuits of commercial monitors:

TABLE 1 MAX. MAX. Input range horizontal Power of power Brand Size frequency consumption supply OPTIQUEST 17″ 69 KHz 80 W AUTO V775 TATUNG 17″ 94 KHz 105 W  AUTO 17N APPLE 15″ 48 KHz 70 W FULL M2943 TATUNG 15″ 64 KHz 75 W FULL 15VHR

NOTE 1: The circuit control of automatic range is shown in FIG. 1a. If the input voltage of the power supply is 110V, the Triac I₈₀₁ turns on to make the circuit become a voltage doubler. If the power source is 220V the Triac I₁₈₀ cut off, the circuit become bridge rectifier.

NOTE 2: The full range circuit control is shown in FIG. 1b. No matter whether the input voltage of power source is 100V or 220V, the circuit functions as the bridge rectifier.

In the auto range controlling, when the input power source is 110V(i.e. 110V±20% ), the rectifying circuit does the function of a voltage doubler, so that the output voltage is the same as that from a 220V power source (i.e. 220V±20% ). Now referring to FIG. 1a, the voltage value of Vc is in the range of {square root over (2)}×220V×(1±20% )=249V˜373V. This value is different from that in the full range, in which VC=124V˜373V. When the input voltage of the power source is 100V, the current consumption in the full range is approximately twice that in the auto range. For, example, in the OPTIQUEST V775 with power consumption of 80 W, the former has a current of 1.06 A, while the latter has a current of 0.53 A. However, when the input voltage of the power source is 220V, both have a power consumption of 0.53 A. As shown in Table 2, assuming that the power consumption is 80 W for the two circuits, the specifications of the related components are listed as follows

TABLE 2 Specification control Auto range Full range circuits (A) (B) Description Power Q805 = Q805 = When input is 110 V, the transistor 4A/500 V 8A/500 V current of B circuit is twice of that of A Circuit. Filtering C809 = C808 = Since Cv = it , when 110 V capacitor 150μ/200 V 300μ/400 V power input, the voltage of B C810 = circuit will be halved, while 150μ/200 V when i is doubled, then the voltage of C is four times of that of A circuit. output end D811 = D811 = The number of windings in rectifier 100 V 1A 200 V 1A the primary terminals of A/B D812 = D812 = circuits are 116 and 231, 200 V 1A 400 V 1A respectively, while in the secondary terminal, A/B circuits has the same winding numbers. That is in the primary terminal, the ratio of winding number A/B is 0.5. When input is 220 V, the reverse voltage of B circuit is twice of that of A circuit. Voltage 1802 = unnecessary When input is 110 V, the A multiplier 10A/500 V circuit need a voltage and switch (STR81145A) multiplier and switch.

In table II, for the sizes of the parts used in the A and B circuits, the larger the size, the more expensive the cost. Thus the cost is the primary concern in selecting a circuit. After analyzing, it is known that the power consumption is smaller than 80 W, the circuit has an economic cost by using the full range control, and vice versa.

But from the viewpoint of energy saving, no matter whether the control circuit is operated in full range or in auto range, the prior circuit structures are not preferable. Following are the explanations:

For the circuit operating in auto range: referring to FIG. 2a, a voltage multiplying and switch device(I₈₀₁), such as integrated circuit STR80145A. is used. Since a triple directional silicon control rectifier (Triac) is built therein and a voltage decreasing resistor is connected, some loss will occur, as shown in FIG. 2b.

For the circuit operating in full range: in the highest efficiency, the voltage is in a middle range between 88V˜264V; this range is between the range of the 132V˜176V. When the voltage of power supply is closer to an upper limit or a lower limit, the power consumption is larger and the efficiency will become worse.

Table 3 may prove the above description. For example, for a monitor operated in auto range (17 inches monitor), when the power supply is AC110V, the average power consumption is larger than that in of a power supply of AC220V with a value of ${{\frac{66.6 + 67.1 + 97.6 + 94.7}{4}W} - {\frac{64.5 + 66.3 + 91.4 + 92.2}{4}W}} = {2.9\quad W}$

Also, for a for monitor operated in full range (15 inches monitor) with a lower limit of 88V and an upper limit of 264V, then the average power consumption is larger than that operated in the range of AC132V˜AC176V with a value of ${{\frac{61.5 + 63.0 + 59.2 + 59.7}{4}W} - {\frac{59.8 + 60.0 + 57.6 + 57.7}{4}W}} = {2.1\quad W}$

TABLE 3 Pattern cross hatch;  Timing: F_(H) = 31.5 KHz;  F_(V) = 70 Hz Input power supply voltage Specification AC110 V AC220 V Max. Lower Upper Lower Upper Size horizontal limit limit limit limit Brand (inch) freq. Rectifier 88 V 132 V 176 V 264 V OPTIQUEST 17″ 69 KHz Auto range 1.097 A 0.769 A 0.554 A 0.369 A V775 66.6 W 67.1 W 64.5 W 66.3 W TATUNG 17″ 94 KHz Auto range 1.503 A 1.028 A 0.746 A 0.499 A 17N 97.6 W 94.7 W 91.4 W 92.2 W APPLE 15″ 48 KHz Full range 1.025 A 0.699 A 0.517 A 0.353 A M2943 61.5 W 59.8 W 60.0 W 63.0 W TATUNG 15″ 64 KHz Full range 0.956 A 0.657 A 0.488 A 0.335 A 15VHR 59.2 W 57.6 W 57.7 W 59.7 W

SUMMARY OF THE INVENTION

The object of the present invention is to provide a low loss and power saving automatic switching device for the input range of the power source of a monitor.

For a monitor, the power consumption of 2˜3 W almost may be neglected. However, through calculating, it has been discovered that this power is 17% of the power consumption of a monitor. Then, if the power consumption all over the world are added together, it will be appreciated that the total amount is a very large number. Thus, through a long period of research, the inventor of the present invention has derived the following results:

1. The input of a power source is operated in auto range so that the working point of the transformer is within the range of {square root over (2)}×220V(1±20%) for increasing the working efficiency.

2. The Triac within the voltage multiplying, rectifying and switching device are changed to be controlled by the contacts of a relay for dissipating the heat consumption V_(F)×I_(F) (V_(F) is the forward voltage of the Triac in the integrated circuit; I_(F) is the forward current of the Triac in the integrated circuit) induced from voltage multiplying and rectifying.

3. The working current of the voltage multiplying and rectifying device is changed so that it is supplied from the secondary terminal of the transformer for decreasing the power consumption generated from the voltage decreasing circuit.

The TATUNG 17N shown in Table 1 is used as an example, in which the TATUNG 17N has been improved by the above three results. The power consumption is listed in Table 4:

TABLE 4 Power Input Horizontal 110 V level 220 V level Comparing frequency 88 V 132 V 176 V 264 V Before change 31 KHz 97.6 W 94.7 W 91.4 W 92.2 W 94 KHz 105.5 W 101.2 W 97.7 W 99.2 W After change 31 KHz 92.7 W 91.0 W 89.0 W 88.9 W 94 KHz 101.2 W 98.0 W 95.2 W 96.1 W Power consumption before and after changing Result 31 KHz 4.9 W 3.7 W 2.4 W 3.3 W (power saved) 94 KHz 4.3 W 3.2 W 2.5 W 3.1 W

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a rectifier circuit of a prior art monitor the power source of which is controlled by an auto range mode;

FIG. 1b shows a rectifier circuit of a prior art monitor the power source of which is controlled by a full range mode;

FIG. 2a shows the circuit structure of the integrated circuit of a voltage doubler;

FIG. 2b shows the P_(T)-I_(T) characteristic diagram as the integrated circuit STR80145A is conducted;

FIG. 3 shows the block diagram of a monitor;

FIG. 4a shows the circuit structure of the integrated circuit UC3842A used in a switchmode power supply;

FIG. 4b shows a Vcc-Icc characteristic diagram of the integrated circuit UC3842A;

FIG. 5a shows a circuit diagram of the full range control of a switchmode power supply;

FIG. 5b shows a circuit diagram of the auto range control of a switchmode power supply; and

FIG. 6 shows the circuit diagram of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3, the monitor of the present invention comprises a power supply 10, a synchronous signal processor 20, a microprocessor 25, a deflection circuit 30, a deflection drive circuit 35, a high voltage circuit 40, a video pre-amplifier 50 and a video output amplifier 55. The primary advantage of the present invention is to control the action of a power supply 10. In the embodiment of the present invention, the power supply 10 is a switchmode power supply 10. The operation theory of the synchronous signal processor 20, the microprocessor 25, the deflection circuit 30, the deflection drive circuit 35, the high voltage circuit 40, the video pre-amplifier 50 and the video output amplifier 55 are well known by those skilled in the art, and thus not be described herein.

The operation of the power supply 10 of the present invention will be described in the following:

(1) The integrated circuit I₈₀₂ is UC3842A, used by the switchmode power supply 10 of the present embodiment, and the applied embodiment thereof is shown in FIG. 5a. The integrated circuit I₈₀₂ is a high performance current mode controller.

(2) Rectifying Circuit: The alternating current (A.C.) power supply is connected to the rectifier D₈₀₁ through the resistor R₈₀₃ for rectifying, then through a capacitor C₈₀₉, the circuit being further divided into a start-up circuit and a driving circuit.

(3) Start-Lip Circuit: The capacitor C₈₁₅ is charged through a resistor R₈₁₁ by a current. When the voltage of the capacitor C₈₁₅ has increased to 16V, the capacitor C₈₁₅ is connected with the seventh pin of the integrated circuit I₈₀₂ so that the integrated circuit I₈₀₂ will be activated. Then the pulsed modulated signal (PWM) will be sent out from the sixth pin. The activating current of the integrated circuit I₈₀₂ is smaller than 1 mA, and the working current is smaller than 17 mA, referring to FIG. 4b. It is appreciated that after the integrated circuit I₈₀₂ has been activated, when the working current is larger than the charging current of the resistor R₈₁₁, if the current of the diode D₈₁₀ does not feedback, then when the terminal current of the capacitor C₈₁₅ has reduced to below 10V, the integrated circuit I₈₀₂ will be idle.

(4) Driving Circuit: When the pulse sent out from the sixth pin of integrated circuit I₈₀₂ has transferred to a transistor Q₈₀₅ through the resistor R₈₁₅, if the pulse is in high voltage, the transistor Q₈₀₅ will conduct, and vice versa. When transistor Q₈₀₅ is conducted, the current will return to capacitor C₈₀₉ through path →T₈₀₁→Q₈₀₅→R₈₃₁, and the primary inductor L of the transformer T₈₀₁ will store the energy induced from ½·Li² in the coils of the windings. After the transistor Q₈₀₅ has been cut off, then the magnetic energy is converted into electric energy which will be released from the secondary terminal of the transformer so that each set of the power sources can generate voltage.

(5) Voltage Feedback: The voltage generated by the feedback secondary winding of the transformer T₈₀₁ is used to charge the capacitor C₈₁₅ through rectifying of a diode D₈₁₂, filtering of a capacitor C₈₁₆ and a diode D₈₁₅ (the detail may refer to the description of the activating circuit), so that the voltage of the capacitor C₈₁₅ may stay with the range of 10V˜35V (set to be 12V) for operating the integrated circuit I₈₀₂. Then it is connected to the voltage divider of the resistor R₈₂₅ and resistor R₈₂₆ for connecting to the second pin of the integrated circuit I₈₀₂. When the dividing voltage is larger than 2.5V, the sixth pin of the integrated circuit I₈₀₂ will stop outputting voltage immediately. Thereby, the output voltages of all the power supply outputs are controlled to the predetermined value.

(6) Protection of Current Limitation: When the transistor Q₈₀₅ is activating, in order to avoid an over-current phenomenon, a resistor R₈₃₁ is connected in series with the source so that the voltage decrement of the I_(S)×R₈₃₁ is smaller than 1V. Otherwise, the overload of the third pin of the integrated circuit I₈₀₂ will force the integrated circuit I₈₀₂ to stop. The current I_(S) is the source current of the transistor Q₈₀₅.

(7) Frequency Control: When the integrated circuit I₈₀₂ is working, the eighth pin will generate a reference voltage 5V used by the resistor R₈₁₇ and capacitor C₈₂₀ oscillating circuit. The oscillating frequency is determined by the resistor R₈₁₇ and the capacitor C₈₂₀. If the resistor R₈₁₇=4.3K and the capacitor C₈₂₀=0.022 μF, from the specification of the integrated circuit I₈₀₂, it is known that the frequency is approximately equal to 20 KHz.

(8) Error Compensation: There is an amplifier in the integrated circuit I₈₀₂. The gain of the amplifier is controlled by a resistor R₈₂₃, while the reaction is controlled by a capacitor C₈₁₈. Since the feedback value of the voltage in the second pin of the integrated circuit I₈₀₂ is equal to 2.5V±Δυ′ Δυ=error value, the value of Δ υ is directly proportional to the voltage error of each set of power supply outputs. In other words, if the voltage value is larger, then the error of the output voltage for each set of power supplies of the transformer T₈₀₁ is large, and if the error is smaller, then the working voltage of the integrated circuit I₈₀₂ will become short so that the power loss of the transistor Q₈₀₅ is large and the temperature will increase. The error of the output voltage is negatively proportional to the value of the resistor R₈₂₃, but is positively proportional to the value of the capacitor C₈₁₈.

(9) Elimination of Inducing Voltage: Since the transformer T₈₀₁ is an inductive load, when the transistor Q₈₀₅ is cut off from conduction, the primary terminal of the transformer T₈₀₁ will generate a reverse electromotive force which is formed by L×di/dt, wherein L is the inductance of primary terminal of transformer T₈₀₁, i is the induction current of transistor Q₈₀₅; and t is the fall time for cutting off the transistor Q₈₀₅. This inducing voltage has a value of several volts and, thus it exceeds the breakdown voltage of Q₈₀₅ so that the transistor will be destroyed. Therefore, a snubber circuit is needed for absorbing the energy from the electromotive force formed by L×di/dt, so that the voltage will decrease to a value within the range which may be endured by the transistor. The capacitor C₈₁₄ resistor R₈₃₃ and diode D₈₀₇ form one eliminating circuit, and another elimination circuit is formed by the capacitor C₈₁₃ resistor R₈₁₃ and diode D₈₀₆.

(10) Voltage Doubler: Referring to FIG. 5a, (circuit diagram of the switchmode power supply in a full range mode) and FIG. 5b (circuit diagram of the switchmode power supply in an auto range mode), a voltage doubler circuit 13 is further added, which is formed by a voltage multiplying, rectifying and switching device (integrated circuit ) I₈₀₁ and a voltage biasing circuit. If the voltage of the power supply is between 88V˜132V, then the detecting circuit within the voltage multiplying, rectifying and switching device I₈₀₁ will activate and cause the Triac to conduct so that the second pin and third pin of the voltage multiplying, rectifying and switching device I₈₀₁ is shorted to become a connecting line for voltage multiplying and rectifying. If the voltage of the power supply is between 176V˜264V, then the detecting circuit within the voltage multiplying, rectifying and switching device I₈₀₁ will cut off and cause the Triac to stop so that the second pin and third pin of the voltage multiplying, rectifying and switching device I₈₀₁ are opened and only have the function of full wave rectifying.

The characteristic of the present invention is that the voltages of power sources are compared, then the results are outputted to drive a relay SR₈₀₃. By the opening or closing of the contacts of the relay SR₈₀₃, the rectifying circuit 13 may be controlled to output a voltage of one time or two times. Referring now to FIG. 6, the following will describe the voltage multiplying and rectifying circuit 13.

In this embodiment, the type of integrated circuit used is LM339 in which four comparators A, B, C, and D are built. The comparators B, C, and D are connected in parallel with connecting lines for increasing the number of output ports and sink current so as to drive the relay SR₈₀₃ to activate. Thus, the comparators B, C, and D are considered as one unit (in the following call as “comparator BCD”). Also, the voltage of the first pin of the integrated circuit I₈₀₁ is defined as V₁, and the voltage of the second pin of the integrated circuit I₈₀₁ is defined as V₂, and so on.

(a) The initial condition of the input voltage of power source:

The power is input from P805A. When the power source switch S₈₀₁ is closed, then the current will flow through a rectifier D₈₀₁ to resistor R₈₁₁ and resistor R₈₁₂ to charge the capacitor C₈₁₅. When the terminal voltage of the capacitor C₈₁₅ is 16V, the integrated circuit I₈₀₂ will be activated. Then, the feedback voltage of the secondary terminal of the transformer T₈₀₁ will charge the capacitor C₈₁₆ through a diode D₈₁₂. Assuming the terminal voltage of the capacitor C₈₁₆ is 2.5V. In fact, during initially charging the capacitor C₈₁₅, there is voltage existed on the sixth pin of the integrated circuit I₈₀₁ but the third pin (Vcc) still has no voltage. In order to avoid the voltage in the input port larger than that in the power source port to destroy the component, therefore, a diode D₈₀₄ is further added to Vcc for clamping in order to protect the component. Therefore, in the initial condition of the comparator A, since V₆>V₇, thus V₁=0V. When the capacitor C₈₁₆ is charged to a full voltage—12.5V ′ since V₄=V₁=0V, V₅=V₇ and V₇>0V ′ the output voltage of the comparator BCD is 12.5V. Thereby the relay SR₈₀₃ will not operate.

(b) The input of the voltage of power source has remained in a steady condition:

The resistor R₈₀₈ is the forward feedback resistor of comparator A for forming a hysteresis comparing effect. Since in the initial condition, V₁=0V, the resistor R₈₀₈ and resistor R₈₃₈ will be forced to connect in parallel (represent by R₈₀₈//R₈₃₈). That is: $V_{7} = {12.5\quad V \times \frac{R_{808}//R_{838}}{R_{808}//{R_{838} + R_{839}}}}$

It is know from FIG. 6:

R ₈₀₈ =R ₈₃₈ =R ₈₃₉=100K, so that V ₇=4.17V.

$V_{6} = {\sqrt{2} \times V_{AC} \times \frac{R_{810}}{R_{810} + R_{812} + R_{813}}}$

As shown in FIG. 6; it is known that R₈₁₀=100K ′ R₈₁₂=2.7M ′ R₈₁₃=2M ′ if

(I). V_(AC)=100V level ′ then V₆=3.24±0.65V=3.89V˜2.49V so that V₆<V₇, and V₁=12.5V. Thereby, resistor R₈₀₈ is serially connected with resistor R₈₀₇ to Vcc, and similarly connected with R₈₃₉ in parallel. The voltage V₇ is increased to 8.34V so to form a delaying function (referring to the description in (c)). Meanwhile, in comparator BCD, V₄=V₁=12.5V ′ while V₅=V₇=8.34V, V₄>V₅, causing that V₂=0V, i.e., the relay SR₈₀₃ is conducted. Before the contacts of the relay SR₈₀₃ have been closed, since when V₂=0V, the resistor R₈₀₉ will be enforce to short, so that the resistor R₈₀₉ and the resistor R₈₁₀ are virtually in parallel for reducing one half of the voltage V₆ in order to avoid that after the contacts of the relay SR₈₀₃ are closed, the voltage multiplying and rectifying are formed so that the voltage of V₆ is increased to two times, and thus the contacts will open and close alternatively. Thus in design, it is needed that after the relay has been conducted, the voltage V₆ is still unchanged, that is: $V_{6} = {\sqrt{2} \times V_{AC} \times \frac{R_{809}//R_{810}}{R_{809}//{R_{810} + R_{812} + R_{813}}}}$

 since V_(AC)=110V level, resistor R₈₀₉=100K, and thus V₆=3.89V˜2.49V ′ to still remain V₆<V, so that the relay SR₈₀₃ will conduct continuously.

(II). V_(AC)=220V level, then V₆=6.48±1.30V=7.78V˜5.18V, so that V₆>V₇ and V₁ remain unchanged, i.e. V₁=0V. Since V₄=V₁=0V, while V₅ V₇=4.17V, thus V₄<V₅. The output of comparator BCD is 12.5V, and thus the relay SR₈₀₃ will not operate.

(C) The input of the voltage of power source is unstable:

When the voltage of power source is divided as 110V and 220V, then if the power source is abnormal due to power system fault, the voltage of power source is probably between 132V˜176V, and therefore:

(I). When the voltage of power source increases to 132V from 88V (but not attain 176V), two times voltage rectifying and control are used.

(II). When the voltage of power source decreases to 176V from 264V (but not return to 132V), one time voltage rectifying and control are used.

The overlapping region between 132V and 176V could be controlled by the delay characteristic of this comparator. That is, if the resistance of the resistor R₈₀₈ is large, then the voltage range of the overlapping region is also large, and vice versa. In addition, since the resistor R₈₀₉ is affected by diode D₈₀₃, only one side of delay characteristic is affected, i. e., it is affected only to the extent that the voltage is increased from 132V. According to the measuring results shown in FIG. 6, when the voltage is increased from 88V to 194V, the system is in the state of two times rectifying control, while when the voltage is decreased from 264V to 148V, the system is in the state of one time rectifying control.

The contact resistance of the contacts of the relay SR₈₀₃ is very small, generally smaller than 0.1 Ω. For, example, for the TATUNG in the Table 3, in AC110V, the current consumption is between 1.503A˜1.028A, and thus power consumption I²R is equal to 0.226 W˜0.106 W. For the same current, the power consumption in the prior art circuit driving portion (Triac) is larger than 1.5 W˜1.0 W (referring to FIG. 2b). Moreover, when the input is AC110 level, the relay SR₈₀₃ will operate (specification of coil: DC12V, 400 Ω). The power consumption of the voltage biasing portion thereof is 12²/400=0.36V, while the power consumption of the voltage biasing portion in the prior-art circuit is 0.84 W˜2.73 W (testing values). The following table (Table 5) lists the power consumption in the two conditions:

TABLE 5 Input of power source AC110 V level AC220 V level control circuit 88 V 132 V 176 V 264 V Prior art Biasing 0.84 W 1.62 W 1.13 W 2.73 W (STR80145A) portion Driving >1.5 W >1.0 W 0 W 0 W portion Present Biasing 0.36 W 0.36 W 0 W 0 W embodiment portion (LM339 + Driving <0.226 W <0.106 W 0 W 0 W Relay) portion Power saved >1.754 W >2.154 W 1.13 W 2.73 W (results)

In summary, it is to be appreciated that from the measuring value of Table 4 or from the analyzing value of Table 5, that the automatic switching device for the power source input range of the present invention saves more power than that in the prior art circuit. 

What is claimed is:
 1. A device for automatically switching a power source input range of a monitor used with a personal computer, said device being installed in a switchmode power source circuit of the monitor, comprising: a bridge rectifier for rectifying an alternating current power source into a direct current power source for supplying power to internal circuits of the monitor; two serial capacitors having positive ends connected with positive ends of said bridge rectifier, and negative ends connected with negative ends of said bridge rectifier, so as to filter output pulses produced by bridge rectifier, thereby reducing ripple current; a relay for controlling said bridge rectifier to output one times or two times a voltage of the direct current power source, said relay having two contacts, one being electrically connected with a connection point of said serial capacitors, and the other being electrically connected with a power source line so that when the power source line has a power input of AC 110V, said two contacts conduct and said bridge rectifier outputs two times the voltage of the direct current power source; and a comparator arranged to detect the voltage level of said direct current power source, the output of said comparator being arranged to drive said relay so as to turn on or turn off said contacts of said relay; wherein when the alternating current power source input is at a 100V level, said device for automatically switching the power source input range is arranged to be automatically switched to control said bridge rectifier to output one times the voltage of the direct current power source; while when the alternating current power source input is at a 220V level, said device for automatically switching the power source input range is arranged to be automatically switched to control said bridge rectifier to output two times the voltage of the direct current power source.
 2. The device for automatically switching a power source input range of a monitor used with a personal computer as described in claim 1, further comprising a voltage biasing circuit, the power source of said voltage biasing circuit being supplied from the secondary winding of a power supply. 